A new genus, Seulocia, is established for Leucosia species with segments 3-6 of the male abdomen fused, and the first male pleopod with straight shaft, twisted once on its axis: L. laevimana Miers, 1884, L. latirostrata Shen & Chen, 1978, L. pubescens Miers, 1877, L. pulchra Shen & Chen, 1978, L. rhomboidalis de Haan, 1841, L. truncata Alcock, 1896, and L. vittata Stimpson, 1858, and three new species: S. anahita, S. crepuscula, S. cristata. The species are described or redescribed and illustrated, extended synonymies are given, and a key for their identification is provided. The choice of the lectotype of L. rhomboidalis is discussed.